Fluid working machines are generally used, when fluids are to be pumped or fluids are used to drive the fluid working machine in a motoring mode. The word “fluid” can relate to both gases and liquids. Of course, fluid can even relate to a mixture of gas and liquid and furthermore to a supercritical fluid, where no distinction between gas and liquid can be made anymore.
In particular, such fluid working machines are used, if the pressure level of a fluid has to be increased. For example, such a fluid working machine could be an air compressor or a hydraulic pump.
Generally, fluid working machines comprise one or more working chambers of a cyclically changing volume. Usually for each cyclically changing volume, there is provided a fluid inlet valve and a fluid outlet valve.
Traditionally, the fluid inlet valves and the fluid outlet valves are passive valves. When the volume of a certain working chamber increases, its fluid inlet valve opens, while its fluid outlet valve closes, due to the pressure differences, caused by the volume increase of the working chamber. During the phase, in which the volume of the working chamber decreases again, the fluid inlet valve closes, while the fluid outlet valve opens due to the changed pressure differences.
A relatively new and promising approach for improving fluid working machines are the so-called synthetically commutated hydraulic pumps, also known as digital displacement pumps or as variable displacement pumps. Such synthetically commutated hydraulic pumps are known, for example, from EP 0494236 B1 or WO 91/05163 A1. In these pumps, the passive inlet valves are replaced by electrically actuated inlet valves. Preferably the passive outlet valves are also replaced by electrically actuated outlet valves. By appropriately controlling the valves, a full-stroke pumping mode, an empty-cycle pumping mode (idle mode) and a part-stroke pumping mode can be achieved. Furthermore, if both inlet and outlet valves are electrically actuated, the pump can be used as an hydraulic motor as well. If the pump is run as a hydraulic motor, full-stroke motoring and part-stroke motoring is possible as well.
A major advantage of such synthetically commutated hydraulic pumps is their higher efficiency, as compared to traditional hydraulic pumps. Furthermore, because the valves are electrically actuated, the output characteristics of a synthetically commutated hydraulic pump can be changed very quickly.
For adapting the fluid flow output of a synthetically commutated hydraulic pump according to a given demand, several approaches are known in the state of the art.
It is possible to switch the synthetically commutated hydraulic pump to a full-stroke pumping mode for a certain time, for example. When the synthetically commutated pump runs in a pumping mode, a high pressure fluid reservoir is filled with fluid. Once a certain pressure level is reached, the synthetically commutated pump is switched to an idle mode and the fluid flow demand is supplied by the high pressure fluid reservoir. As soon as the high pressure fluid reservoir reaches a certain lower threshold level, the synthetically commutated hydraulic pump is switched on again.
This approach, however, necessitates a relatively large high pressure fluid reservoir. Such a high pressure fluid reservoir is expensive, occupies a large volume and is quite heavy. Furthermore, a certain variation in the output pressure will occur.
So far, the most advanced proposal for adapting the output fluid flow of a synthetically commutated hydraulic pump according to a given demand is described in EP 1 537 333 B1. Here, it is proposed to use a combination of an idle mode, a part-stroke pumping mode and a full-stroke pumping mode. In the idle mode, no effective pumping is done by the respective working chambers during their working cycle. In the full-stroke mode, all of the usable volume of the working chamber is used for pumping fluid to the high-pressure side within the respective cycle. In the part stroke mode, only a part of the usable volume is used for pumping fluid to the high-pressure side in the respective cycle. The different modes are distributed among several chambers and/or among several successive cycles in a way, that the time averaged effective flow rate of fluid through the machine satifies a given demand.
In addition to these previously known controlling methods, different basic controlling strategies can be applied as well. In fact, some additional basic controlling strategies have been conceived by the inventors already. Such additional basic controlling methods will be described in detail in the following.
In the past, synthetically commutated hydraulic pumps were controlled in a way, that a certain basic control strategy has been selected and employed over the whole range of working conditions of the synthetically commutated hydraulic pump. So far, improvements in controlling synthetically commutated hydraulic pumps have been performed by modifying an existing control strategy or by introducing a new basic control strategy and applying the respective idea to the whole range of working conditions of the synthetically commutated hydraulic pump. For example, the controlling method described in EP 1 537 333 B1 is applied for all working conditions of the synthetically commutated hydraulic pump.
Of course, it is straight forward and relatively easy to implement a certain basic control strategy over the whole range of working conditions of a synthetically commutated hydraulic pump. Also, one has to admit, that such a synthetically commutated hydraulic pump already works quite well.
However, so-far proposed methods still have draw-backs and certain limitations. A major issue is the problem of pressure pulsation. Especially under certain working conditions, huge variations in the fluid output flow of the fluid working machine can occur. This results in pressure pulsations, which are unwanted. Such pressure pulsations are noticeable by the operator of a hydraulic machine, powered by the synthetically commutated hydraulic pump. For example, the operator can notice a start-stop-behaviour of a hydraulic cylinder (“stiction” effect). The pressure pulsation can even lead to an increased wear and ultimately to the destruction of components of the hydraulic circuit.
Another problem is the time responsiveness, i. e., the time, the fluid working machine needs after a change in fluid flow demand to adjust its fluid flow output. This time delay can be quite long, especially under certain working conditions. Of course, it is unwanted, that the operator of a machine has to wait for a noticeable time interval, after he has changed the demand.
As an example, the method described in EP 1 537 333 B1 will be further explained. According to this method, a certain, previously defined volume fraction is chosen for the part-stroke pumping. For real applications, the applicant of EP 1 537 333 B1 has chosen a volume fraction of 16.67% (i.e. ⅙). Admittedly, this control method is suited for fluid flow demands in the region below around 15%. However, if the fluid flow demand is very low, say at 2%, the time intervals between two part-stroke pumping pulses are still quite large. The situation is also quite bad in the region slightly above 16.67%, for example at a fluid flow demand of 17%. Here, the fluid flow demand can be either provided by constantly pumping with a 16% part-stroke pumping cycle and inserting a full-stroke pumping stroke in this series with very large time intervals in-between. It would also be possible to abandon the part-stroke pumping in this regime and to satisfy the demand solely using full-stroke pumping cycles. The time intervals between two consecutive pumping cycles will be much smaller. However, noticeable pulsation will still occur.